This invention generally relates to wideband communication systems. In particular, the invention relates to image suppression in received wideband communications.
Wideband communication systems, such as television and radio communication systems, use a wide frequency spectrum to communicate information. Typically, the wide spectrum is divided into a group of assigned radio frequencies for carrying the information.
In a heterodyne receiver, the received signal is amplified by a radio frequency (RF) amplifier and mixed with an adjustable local oscillator (LO) signal to produce intermediate frequency (IF) signal. One problem with mixer circuits is the generation of image frequency signals.
When two signals are mixed, signal components are produced at the sum and difference of the two signals, and their harmonics. Equation 1 illustrates the potential signal components where FLO is a local oscillator frequency being mixed with a radio frequency, FRF and m and n are integers or zero.±mFRF±nFLO  Equation 1Since input circuits typically have limited selectivity in the mixing process, undesired interference may create harmful products, as illustrated by Equation 2. F1 is an undesired interference frequency and m, n and p are an integer or zero.±mFRF±nFLO±pF1  Equation 2
For example, when an intermediate frequency signal, FRF, is mixed with a local oscillator, FLO, the result will produce signals at two frequencies, FLO+FRF and FLO−FRF. One of the two signals is the desired frequency and the other is at an undesired frequency. Additionally, based on the quality of the mixers, undesirable harmonics, such as FLO+2FRF, could also be produced. In a wideband communication system utilizing multiple frequencies, the frequencies that these undesired signals fall upon may be the same frequency used for another information signal. As a result, the undesired signal produces an image on the desired information signal at that frequency.
One approach to removing the image signal from the desired signal is by filtering. Typically, filtering only reduces the magnitude of the image signal by less than 30 decibels (dB). In some applications, a 30 dB attenuation is not sufficient.
Another approach is to use an image rejection mixer 10, as illustrated in FIG. 1. A received radio frequency signal is input into the image rejection mixer 10. The received signal is input to an in-phase mixer 14 and a quadrature phase mixer 12. A local oscillator (LO) 11 generates a carrier signal. The carrier signal is input into the quadrature phase mixer 12 to produce a demodulated quadrature phase signal and into a 90 degree phase shift device 16, such as a RC-CR circuit, to produce an in-phase carrier. The in-phase carrier is input into the in-phase mixer 14 to produce a demodulated in-phase signal. The demodulated quadrature phase signal is subsequently delayed by a 90 degree phase shift device 22. An adder 20 combines the phase delayed quadrature phase signal to the in-phase signal to produce the desired signal. Typically, the image signal will be out of phase with the desired signal. As a result, the combining cancels the image signal leaving only the desired signal.
To illustrate, WRF is the frequency of the received signal and WLO is the frequency at the local oscillator. The image rejection mixer 10 processes the desired signal as follows. I(t) is the demodulated in-phase signal. Q(t) is the demodulated quadrature phase signal. Q′(t) is the phase delayed quadrature phase signal. O(t) is the subtracted signal.                                                                                                                                I                    ⁡                                          (                      t                      )                                                        =                                                            cos                      ⁡                                              (                                                                              W                            RF                                                    ⁢                          t                                                )                                                              *                                          cos                      ⁡                                              (                                                                              W                            LO                                                    ⁢                          t                                                )                                                                                                                                                                                      =                                                            1                      2                                        ⁡                                          [                                                                                                    cos                            ⁡                                                          (                                                                                                W                                  RF                                                                -                                                                  W                                  LO                                                                                            )                                                                                ⁢                          t                                                +                                                                              cos                            ⁡                                                          (                                                                                                W                                  RF                                                                +                                                                  W                                  LO                                                                                            )                                                                                ⁢                          t                                                                    ]                                                                      ,                                                    ⁢                                  ⁢                                            where              ⁢                                                          ⁢                              W                RF                                      -                          W              LO                                >          0                                    Equation  3            The cos(WRF+WLO)t is removed by low-pass filtering.                                                                                                                                         Q                    ⁡                                          (                      t                      )                                                        =                                                            cos                      ⁡                                              (                                                                              W                            RF                                                    ⁢                          t                                                )                                                              *                                          cos                      ⁡                                              (                                                                                                            W                              LO                                                        ⁢                            t                                                    -                                                      π                            2                                                                          )                                                                                                                                                                    =                                                                            1                      2                                        ⁢                                                                                  ⁢                                          cos                      ⁡                                              [                                                                                                            (                                                                                                W                                  RF                                                                -                                                                  W                                  LO                                                                                            )                                                        ⁢                            t                                                    +                                                      π                            2                                                                          ]                                                                              =                                                            -                                              1                        2                                                              ⁢                                                                                  ⁢                                          sin                      ⁡                                              (                                                                              W                            RF                                                    -                                                      W                            LO                                                                          )                                                              ⁢                    t                                                                                                          Equation  4            Q′(t)=−½sin[(WRF−WLO)t−π/2]=−½cos(WRF−WLO)t  Equation 5O(t)=I(t)+Q′(t)=cos(WRF−WLO)t  Equation 6
The image rejection mixer 10 processes the image signal which is inverted with respect to the desired signal as follows.I(t)=½cos[−(WRF−WLO)t]=½cos(WRF−WLO)t  Equation 7Q(t)=½cos[−WRF−WLO)t+π/2]=½sin(WRF−WLO)t  Equation 8Q′(t)=½sin[(WRF−WLO)t−π/2]=−½cos(WRF−WLO)t  Equation 9O(t)=I(t)+Q′(t)=½cos(WRF−WLO)t−½cos(WRF−WLO)t=0  Equation 10
Accordingly, after image cancellation, the desired signal is recovered, Equation 6, and the image signal is canceled, Equation 10.
Due to variations in the tolerances of the resistors and capacitors in the phase shift device 16 and temperature variations, the phase differential between the carrier and the quadrature carrier may not be maintained at an ideal 90 degrees, degrading performance of the image mixer.
Accordingly, it is desirable to have alternate approaches for image suppression in received wideband signals.